The present invention relates to an organic EL (electroluminescence) display having an organic EL device comprising an organic light emitting layer.
In an organic EL display comprising numeral pixels each constituted by an organic electric field light emitting device (hereinafter referred to as an organic EL device), an electron and a hole are injected to an organic light emitting layer from a cathode and an anode, respectively, by applying a voltage to the organic EL device, and recombination of the electron and the hole occurs in the organic light emitting layer to cause emission of light.
Examples of the organic EL device equipped in an organic EL display include a single hetero type organic EL device shown in FIG. 1. The organic EL device comprises a transparent substrate 1, such as a glass substrate, having thereon an anode 2 comprising a transparent conductive film, such as ITO (indium tin oxide), and having further thereon an organic layer 5 comprising a hole transfer layer 3 and a light emitting layer 4, and a cathode 6 comprising aluminum, in this order.
In the organic EL device having such a constitution, a positive voltage is applied to the anode 2, and a negative voltage is applied to the cathode 6. A hole injected from the cathode 2 reaches the light emitting layer 4 through the hole transfer layer 3, and an electron injected from the cathode 6 reaches the light emitting layer 4, to cause recombination of the electron and the hole to occur in the light emitting layer 4. As a result, light having a prescribed wavelength is generated and emitted from the transparent substrate 1 as shown by the arrow in FIG. 1.
Accordingly, an organic EL display can be formed by arranging a large number of the organic EL devices in a matrix form, as described in the foregoing.
An example of the conventional organic EL display is shown in FIG. 2. The organic EL display shown in FIG. 2 comprises a transparent substrate 7 having thereon transparent electrodes 8 in a stripe form as an anode, and organic layers 11a, 11b, 11c et al. in a stripe form are formed further thereon to cross the transparent electrodes 8 at right angles, and cathodes 12 having the substantially same dimension as the organic layer 11a (11b and 11c) are formed on the organic layers 11a, 11b, 11c et al. The organic layers 11a, 11b and 11c have light emission characteristics corresponding to one of red (R), green (G) and blue (B), respectively, and thus the organic EL display becomes a full-color or multi-color display.
The mode of displaying an image in the color organic EL display shown in FIG. 2 will be described. In the color organic EL display, as shown in FIG. 3, a scanning circuit 13 is connected to the transparent electrodes 8, and a brightness signal circuit 14 is connected to the cathodes 12. A signal voltage varying with time is applied to the organic layers 11a to 11c at the point of intersection between the transparent electrodes 8 and the cathodes 12 by the scanning circuit 13 and the brightness signal circuit 14, and thus the respective organic layers 11a to 11c emit light. By using such a mode of control, the organic EL display can also function as an image reproduction apparatus.
However, the organic EL display involves the following problems.
In the case where the organic EL device having several hundreds scanning lines is driven in a simple matrix mode, an electric current of about 1 A/cm2 is necessary to ensure sufficient brightness. In this case, while depending on the size of the display, an electric current of about from 0.5 to 1 A momentarily flows through the transparent electrodes 8 on the side of the scanning circuit 13.
ITO generally used as the transparent electrodes 8 has a resistance larger than a metal, such as aluminum, and its alloy by about 100 times, and therefore, when a large electric current of about from 0.5 to 1 A flows therein, a large voltage drop occurs in the transparent electrodes 8. When such a large voltage drop occurs in the transparent electrodes 8, the voltage applied to the respective organic EL devices in the organic EL display becomes uneven to considerably deteriorate the display performance of the organic EL display.
In other words, in the case where the display is driven in the single matrix mode, while depending on the size of the display, the electric current flowing the electrode of the scanning side becomes larger than the electric current flowing in the electrode of the brightness signal side by 100 to 1,000 times due to the driving principles thereof. However, in the organic EL display, because a large electric current flows in the transparent electrodes 8 having a large resistance, a large voltage drop occurs in the transparent conductive film constituting the transparent electrodes 8, and the voltage applied to the organic layers 11a, 11b, 11c, et al. constituting the respective pixels becomes uneven, so as to deteriorate the display performance and to increase the consuming electric power in the transparent electrodes 8.
In the color organic EL display shown in FIG. 2, because the organic layers 11a, 11b, 11c, et al. are formed along the lengthwise direction of the cathodes 12 in a stripe form under the whole surface thereof, a brightness signal necessary for each colors, R, G and B, should be applied from the cathodes 12. Therefore, it is necessary that the brightness signal circuit 14 is connected to the cathodes 12, and the scanning circuit 13 is connected to the transparent electrodes 8.
Because the electric power consumed in the transparent electrodes 8 becomes large as described above, the property of low power consumption of the whole organic EL display is impaired. Therefore, in order to obtain an organic EL display of a low electric power consumption, it is necessary to lower the resistance of the electrode of the scanning side to decrease the voltage drop.
As measures of lowering the resistance of the electrode of the scanning side, a technique is described in JP-A-5-307997 in that a metallic wiring is attached to transparent electrodes. According to this technique, a metal with low resistance is provided at a part between the transparent electrodes and the organic layer to lower the resistance of the scanning electrode.
However, in order to sufficiently lower the resistance by this technique, the area of the metallic wiring attached to the transparent electrodes is necessarily made as large as possible. When the area of the metallic wiring is large, it covers the organic layer 11a (11b and 11c) to be a light emission part, and as a result, the light emission area of the organic EL device becomes small to deteriorate the light emission efficiency.
While it is also considered to lower the resistance by increasing the thickness of the metallic film, such measures may cause a short circuit between the anode and the cathode and unevenness in thickness of the organic layer.
The invention has been developed in view of the circumstances described above.
An object of the invention is to provide an organic EL display that realizes low electric power consumption and display with high brightness by decreasing the resistance of the electrode on the scanning side.
The invention relates to a display device comprising a substrate,
first electrodes in a form of a stripe comprising a transparent conductive material formed on the substrate,
an insulating film having prescribed openings formed on the first electrodes,
an organic layer comprising an organic light emitting material formed on the openings and the insulating film, and
second electrodes in a form of a stripe formed on the organic layer,
wherein the first electrodes comprised of a material having a larger resistance than a resistance of the second electrodes,
the first electrodes and the second electrodes are arranged to cross each other,
the organic layer is formed only in the crossing part of the first electrodes and the second electrodes,
a first circuit supplying a first electric current is connected to the first electrodes, and
a second circuit supplying a second electric current larger than the first electric current is connected to the second electrodes.
The substrate may be a transparent substrate.
The second electrode may be comprised of a metal or an alloy.
The openings formed in the insulating layer may have a substantially rectangular shape in a plan view, and may be formed to have a tapered shape, in which the inner wall of the openings is gradually slanted toward the outside from the lower end to the upper end of the inner wall.
The organic layer may be formed in the form of plural independent islands, each of which has a substantially rectangular shape, to fill up the openings, and the width of the organic layer at the upper surface thereof in the width direction of the first electrodes may be larger than the width of the first electrodes.
The second electrodes may be formed to cover the side surface and the upper surface of the organic layer.
In the organic EL display of the invention, because the plural organic layers may be provided in the form of independent substantially rectangular islands, the arrangement and combination of the organic layer with the cathode and the anode can be arbitrarily conducted, and therefore the selection and arrangement of the pixel comprising the organic EL device and the electrode to be a constitutional component of the device. Accordingly, in the case of, for example, a color organic EL display, a second electrodes comprising a metal or an alloy having a small resistance can be used as an electrode of the scanning side, in which a larger electric current flows.
Because the inner wall of the openings formed in the insulating layer may have a tapered shape, in which the inner wall is gradually slanted toward the outside from the lower end to the upper end of the inner wall, the step coverage of the organic layer provided to fill up the openings c an be improved, and thus breakage of the second electrodes formed to cover the organic layer can be prevented.
Furthermore, because the width of the organic layer at the upper surface thereof in the width direction of the first electrodes may be larger than the width of the first electrodes, the contact area of the organic layer with the first electrodes, i.e., the area of the openings at the bottom thereof, can be made large.